This invention relates to a method of manufacturing a semiconductor element which is formed by a thin film process, and more particularly to a method of applying ultraviolet rays in the case where an amorphous semiconductor layer formed on a large substrate is annealed by applying ultraviolet rays to it, to form a polycrystal semiconductor layer.
Recently, there has been a strong demand for provision of a liquid crystal panel which is large in size and high in resolution, or a contact type image sensor which is high in operating speed and high in resolution. In order to meet this requirement, it is essential to uniformly form thin film transistors (TFT) on a large insulating substrate which are high in performance.
The thin film transistor is formed for instance as follows: A polycrystal semiconductor layer is formed on a glass substrate which is low in strain temperature and low in manufacturing cost and can be made large in area with ease. In the polycrystal semiconductor layer thus formed, a semiconductor active region forming a channel is formed, and in addition a source electrode, a drain electrode, and a gate electrode are formed which are low in resistance. In the case where a glass substrate is employed which is low in strain temperature, it is necessary to protect the glass substrate from thermal damage. Hence, the polycrystal semiconductor layer is formed as follows: That is, an amorphous silicon (a-Si) layer is formed on the glass substrate, and is then annealed with the output ultraviolet rays of an excimer laser, to form a polycrystal silicon (poly-Si) layer. With the aid of a beam homogenizer or the like, the output ultraviolet rays of the excimer laser can be formed into a beam which provides a beam spot of about 10 mm square on the glass substrate. The beam spot is smaller than the glass substrate. Therefore, the beam spot is applied to the glass substrate in a scanning mode several times while being shifted in position, to anneal the amorphous semiconductor layer formed large on the glass substrate. In this case, the laser beam is applied to the amorphous semiconductor layer in such a manner that the peripheries of the beam spots are overlapped with one another as indicated at L in FIGS. 4 or 5. This is to eliminate the difficulty that the amorphous semiconductor layer is not completely irradiated with the laser beam, and to eliminate the difficulty that the amorphous semiconductor layer is insufficiently annealed because the energy of irradiation is abruptly decreased at the periphery of the beam spot,
The above-described conventional method is disadvantageous in the following points: It is difficult to make the sum of energy provided by the overlapped portions of the beam spots equal to that provided by the remaining portions. Hence, at the overlapped portions of the beam spots, the film stress may be increased, or the surface flatness may be decreased. That is, it is impossible to form a polycrystal semiconductor layer high in quality according to the conventional method.
Even if the laser beam is processed and shaped with a beam homogenizer or the like, the intensity of energy in the beam is fluctuated in distribution of the order of several percent as shown in FIG. 5, which also makes it difficult to uniformly irradiate the semiconductor layer with the laser beam so as to allow all the parts of the amorphous semiconductor layer to receive the same amount of energy.